Various electronic filter arrangements for processing signals in hearing devices have been suggested over the years. Examples of such arrangements can for example be found in US 2014/0097906 A1, U.S. Pat. No. 6,888,408 B1, GB 2 266 021 A as well as the paper titled “An enhanced slew rate source follower”, IEEE Journal of solid-state circuits (Vol 30, No 2, 1 Feb. 1995, pages 144-146) by Kenney J G et al.
Referring now to US 2014/0097906 A1 an amplifier having a high-pass filter is suggested. However, this high-pass filter has a major drawback in that its cut-off frequency is not constant with an input signal level to the amplifier. Thus, when a large low frequency signal is introduced to the input the amplifier loses partly its high-pass filter function. As a result the low frequency input signal is feed through to the output of the amplifier without the predetermined and constant attenuation. This lack of attenuation will inevitably result in a poor sound quality in the low-frequency range.
U.S. Pat. No. 6,888,408 B1 discloses a high-pass filter suffering from the same problem due to a resistance biasing at the input stage of the amplifier, i.e. bias current varies with output signal level. Moreover, the output noise level of the arrangement suggested in U.S. Pat. No. 6,888,408 B1 is high, because a common source n-channel metal-oxide-semiconductor (NMOS) output stage is used instead of a common source p-channel metal-oxide-semiconductor (PMOS) transistor.
The above-mentioned disadvantages regarding the high-pass filter also apply to GB 2 266 021 A and the paper of Kenney J G et al.
Thus, there seems to be severe disadvantages associated with the high-pass filters suggested in the above-mentioned prior art references.
It may be seen as an object of embodiments of the present invention to provide an amplifier topology that facilitates incorporation of a stable filter stage having a frequency response which is essentially unaffected by a signal level arriving at an input amplifier stage of the amplifier.
It may be seen as a further object of embodiments of the present invention to apply the amplifier in an advantageous configuration in a microphone unit in order to obtain a dual omni-output signal.